1. Field of the Invention
This invention pertains to cathode ray tubes or other electronic devices employing electron beams, and particularly to those cathode ray tubes and electron guns contained therein that are used to display high-resolution imagery.
2. Discussion of Related Art
The principal components of a cathode ray tube (CRT) are (FIG. 1): an envelope 101, an electron gun 102 (which has a “beam-forming region” 103 and a main lens region 104), a deflection yoke 105, and a display screen 106. A typical prior-art electron gun beam-forming region 103 consists of a cathode 107, a first electrode 108 (often called a “Wehnelt” or “suppressor” electrode) with an aperture 109 and a second electrode 110 (often called an “extractor” electrode) with a coaligned aperture 111. The primary function of the beam-forming region is to allow control of the electron beam current, and to establish the emitted electrons along trajectories 114 that allow formation of a small spot on the display screen. The extractor electrode in conjunction with a subsequent electrode may form a pre-focus lens region 112. The main lens region 104 of the electron gun will typically consist of one or more focus electrodes and a final anode electrode 113. The pre-focus lens region 112 and main lens region 104 create a focusing apparatus that bends the trajectories 114 of the electrons emanating from the beam-forming region 103 into converging paths so that they form a small spot on the screen 106. The deflection yoke 105 is used to scan the focused electron beam in a raster or vector-based pattern on the screen 106 to form the display imagery.
Depending upon the end use of the CRT, the electron gun 102 is typically of either mono-beam, as depicted in FIG. 1, or a three-beam type, forming a single spot or three spots, respectively. In a three-beam electron gun, each of the three beams emanates from its own beam-forming region, providing individual control over the current produced in that beam. The three beams may either have individual pre-focus and main lens assemblies, or all of the beams may share a single pre-focus and main lens region. In a three-beam electron gun used in a color display it is common for there to be electromagnetic means for overlapping the three electron beam spots in the same color phosphor trio's location on the display screen.
Mono-beam guns are frequently used in CRTs for projection television displays or monochrome displays. Three-beam electron guns are generally used in CRTs that produce a color display. In this case, additional components (such as a shadow mask) are used to direct the three beams to the appropriate color phosphors on the screen. Main lenses are of three principal types, which are described in U.S. Patent Application Publication No. US 2002/0089277, filed Jan. 5, 2002, which is hereby incorporated by reference herein.
The most important operating characteristics of CRTs are video image brightness, resolution and display size. In a typical CRT, increasing brightness reduces resolution because the electron beam spot size increases at higher electron beam current levels. Increasing the display size without increasing the beam current reduces the video image brightness (per unit area) because the emitted electron beam must cover a larger display area. The resolution of a CRT is determined by the finest spatial intensity changes that can be written to the display screen by the electron beam. Accordingly, the resolution of a CRT is thus determined by both the spot size and the rate at which the electron beam current can be modulated. The electron beam current modulation rate is affected by the speed of the video driver electronics and the voltage range required by the electron gun beam-forming region. To produce a high resolution display in a typical CRT it is necessary to (1) produce a small electron beam spot on the screen, (2) operate the beam-forming region of the electron gun to minimize the voltage range required for beam current modulation, and (3) use video driver electronics that have very fast voltage change capability. In typical prior art CRTs, items (1) and (2) cannot be achieved simultaneously without changes to the electron gun design that would compromise the manufacturing tolerances, and thus increase the cost of the electron gun, and item (3) is costly and causes reduced reliability due to the increased power dissipation in the high-speed electronics.
Prior art CRTs operate such that the main lens of the electron gun converges an initially divergent electron beam to a spot on a display screen. In this mode of operation, the electrons emitted from the cathode are focused together by the beam-forming electrodes into a small region close to the center of the suppressor and extractor electrode apertures, known as the “crossover”. The crossover is a natural consequence of the operation of the suppressor and extractor electrodes as an immersion lens, and exists because of the shape of the electrostatic fields generated in the beam-forming region by the cathode and the beam-forming electrodes. By adjusting the voltages of the electrodes that comprise the main lens of the electron gun, the crossover is positioned in the object plane of the main lens and the display screen is placed in the image plane of the main lens. The focal distance of the main lens is thus adjusted to image the crossover onto the display screen. In this mode of operation, the spot size will be determined by the size of the crossover, which is in turn determined by the size of the electron emission area on the cathode and the electron-optics characteristics of the beam-forming electrodes of the gun.
FIGS. 2A and 2B depict a beam-forming region 200 with thermionic cathodes. Heating of a cathode 201 causes electrons to be emitted at a cathode surface 202. Some of the electrons are pushed back to the cathode surface by a suppressor electrode 203, but an extractor electrode 204 is maintained at a sufficiently positive voltage relative to the suppressor electrode 203 to allow an accelerating electric field to penetrate through a circular optical aperture 205 in the suppressor electrode 203 to the surface of cathode 202. The accelerating electric field extracts electrons from the surface of the cathode 201 in the area where the accelerating electric field exists. This configuration results in a converging electron beam 206 that crosses over the central axis of symmetry 211 at a position between the suppressor electrode 203 and the extractor electrode 204. This position is typically referred to as a “first crossover” 209. For a fixed positive voltage applied to the extractor electrode 204 and a zero or reference voltage applied to the suppressor electrode 203, adjusting the voltage of the cathode surface 202 will cause more or less accelerating electric field penetration to the cathode surface 202. In FIG. 2A, the cathode voltage is less than, but close to the voltage applied to the extractor electrode 204, and is the same as an isopotential contour 207. Isopotential contours less than the potential of the cathode 201, such as an isopotential contour 208, represent an electric field that repels electrons back to the cathode surface 202. Isopotential contours that are greater than the cathode voltage and adjacent to the cathode surface 202 represent an extracting electric field in that region of the cathode surface 202. Since the cathode potential is close to that of the extractor electrode 204, only a small region of the cathode surface 202 is emitting electrons and thus the emitted beam current is small. The shape of the electron trajectories, including the position and the size of the first crossover 209, is determined by the shape of the electric field in the vicinity of the cathode surface 202 and the optical aperture 205. In FIG. 2B, the cathode voltage is lowered to a value greater than but close to the voltage of the suppressor electrode 203, and is equal to the potential of the isopotential contour 208. Because of the larger amount of the cathode surface 202 that is exposed to the extracting electric field, the emitted current is much larger. The beam-forming region 200 thus effectively forms a controllable iris 210 at the cathode surface 202, which controls the emitting area of the cathode. The iris 210 is opened or closed by the varying voltage on the cathode 201. If the voltage of the cathode 201 is brought closer to the voltage on the suppressor electrode 203 then the cathode's active emitting surface becomes larger in diameter. Because of electron-optical aberrations in the immersion lens and transverse thermal velocities of the electrons, the size of the crossover 209 also varies with beam current. The crossover 209 is the object in the electron optical system, in which lenses in the other parts of the electron gun focus the object to form an image on the screen. Therefore, varying the cathode voltage to cause more current to escape from the cathode 201 increases the image size for the optical system, which in turn increases the size of the spot on the screen. Decreasing optical aperture size 205 to obtain a smaller crossover 209 and thus a smaller spot on the screen is limited in effectiveness because higher voltages must be applied to the extractor electrode 204, a larger range of current control voltages must be applied to the cathode 201, and the resulting larger cathode current density, in some cases, may damage the cathode surface 202. Additionally, if the voltage on the extractor electrode 204 is increased, this increases the cathode voltage required to completely turn off the electron beam 206. This causes the active cathode surface area to decrease in size, which in turn decreases crossover size, and thus spot size, but it also reduces the slope of the current versus biasing voltage curve (the “drive curve”). Increasing the voltage on the extraction electrode 204 also increases the angle at which electrons in the beam 206 leave the cathode 201, which may then require an additional focus electrode to be required in the electron gun, increasing its cost. In practice, the trade-off between spot size on the screen and the drive curve necessary for the required electron current is made in accordance with the needs of the equipment employing the electron gun. In general, in a prior-art electron gun, if a smaller-slope drive curve is required to increase beam current from the cut-off value to full current, less electrical power will be required to drive the electron gun, but the spot size will be larger.
Typically in a CRT, the electron beam current which is associated with a dark screen is on the order of 1 microampere and the electron beam current associated with a fully bright screen is on the order of 1 to 2 milliamperes. That factor of 1,000 change in beam current over the useful drive range of the display requires a large voltage change to be applied to the cathode in order to switch the beam current from that appropriate for a dark screen to the beam current appropriate for maximum brightness. For standard NTSC television signals, the frequency components associated with the video brightness extend to approximately 7 megahertz. In a high definition television the situation is more stressful because the beam current must be modulated by applying the same large cathode voltage changes at frequencies in the range of 100 megahertz. The power requirement to modulate the beam current at these frequencies can be large and is an important consideration in the design of a CRT for high definition television.
Prior art monochrome and color electron guns operate with a single electron beam and three electron beams, respectively. In these guns, each of the beams passes through a single aperture in each of the electrodes making up the beam-forming region (as in FIG. 1). Although it is possible to vary the aperture diameters in the beam-forming electrodes, and to also vary the spacing of the electrodes, restricting these variations to practical values limited by manufacturing and positioning tolerance makes only moderate changes to the spot size, drive range, and maximum beam current. An electron gun having multiple apertures in the first and second electrodes of the gun is disclosed in Publication Number U.S. 2002/0089277 (incorporated by reference). In the electron gun disclosed, electrons emitted from a cathode surface pass through the multiple apertures in two beam-forming electrodes and are then converged into a single high current beam by a pre-focusing lens. The high current single beam then passes through a main lens, which may focus the beam onto a display screen of a CRT. The disclosed electron gun has an improved drive curve relative to prior art CRTs, with no degradation in spot size.
Patent Application Publication No. 2002/0167260 discloses an electron gun assembly wherein the first and second electrodes include a plurality of beam passage apertures, which are aligned on each the first and second beam-forming electrodes in a direction perpendicular to a direction in which an electron beam is scanned. This application describes a means of producing an elliptical spot on the screen that is suitable for specialized color displays that do not have a shadow mask but use a single electron beam to provide information for all colors. In this application the inventors seek to use a plurality of beam passage apertures instead of a single rectangular or elliptical aperture in the beam-forming electrodes. Their claim is that this provides better control over the shape of the desired elliptical spot. The inventors do not use the beam passage apertures to collimate the electron beam, nor is the main lens focused such that the size of the spot on the screen is minimized. In addition, the application does not teach the benefits of such a structure for reducing the drive range of the CRT.
What is needed is an improved beam-forming assembly, improved electron gun, and improved cathode-ray tube to allow the display of high-resolution imagery without spot size increase with increasing electron beam current. The electron gun should also allow lower consumption of electrical power in high-frequency video modulation CRTs, such as used in high definition television. Also, the electron gun should provide lower current load per unit area of the cathode. Methods for manufacturing the beam-forming region and electron gun are also needed.